Since after the Second World War, as mechanized agriculture has increasingly become common in the United States, many different types of granular materials have increasingly been applied to farm fields to assist in crop production. The more common types of such granular material would include various formulations of N (nitrogen) - P (phosphate) - K (potash) combinations that are usally denoted by their percentage of active ingredients such as 5-20-20. These are the most common types of designations of farm fertilizer blends even now and these mixtures are prepared at various fertilizer blending plants by comminuting the various basic materials that go into these combinations and then blending them together to form a more or less uniform mixture of the granular fertilizer. Even these mixtures do not have particles of uniform size or weight. As agriculture has advanced even further, herbicides have been compounded in granular form and, depending upon the manufacturer and industry standards, these particulates tend to also have different sizes and weights. Furthermore, soil conditioners such as gypsum and Ph adjusters such as powdered lime, are also sometimes blended in with the basic fertilizer - herbicide mix.
In short, modern agriculture uses a great variety of particulates that must be applied as uniformly as possible to the field to be cultivated.
The difficulty of spreading what is basically a non-uniform mixture, in a uniform manner, over vast acreages has both fascinated and confounded agricultural equipment manufacturers for many years.
An early solution was proposed by Biszantz, as shown in U.S. Pat. No. 2,538,961, wherein a series of paddles were connected to transversely disposed chains inside a trough, all of which overlaid a diagonally cut table, and material was distributed transversely of the hopper truck as the truck moved across the field. The particular system had two significant drawbacks, the first being that the top flight of paddles attached to the conveyor moved the material outwardly before it could commence its fall past the tapered table, and as it commenced that fall, it was further impacted by the lower returning conveyor belt beneath the tapered table, all of which tended to make non-uniform the already non-uniform mixture, and further, because the granular material thus deposited upon the agricultural fields left a wide center strip behind the spreader truck that was not covered with the granular material. These two factors, namely the non-uniformity of spreading of the material, and the uncovered center strip behind the spreader truck, both combined to render this particular construction unsatisfactory and, in fact, even though it was patented in 1951, it never has come into widespread use in the agricultural community.
Another device used by fertilizer spreader trucks has been the rotating fan such as shown in Hurt, U.S. Pat. No. 2,947,544, which, simply stated, permits the granular material to fall upon a rotating table with vanes disposed thereabove, and the vanes impart a peripheral momentum to such material, and it is dispersed widely to either side and behind the truck to cover the area that the truck has traversed. This is probably the most common expedient used today, not just the Hurt device, but devices of that general rotating table type.
Other expedients have been proposed, as for example in Hamnes, U.S. Pat. No. 3,756,509, which shows tranversely disposed troughs having screw conveyors therein which convey material from a central hopper outwardly, with a uniform drop being attempted by means of spaced openings in the bottom of the trough. This type of device is both costly and subject to "gumming up" and has not found widespread acceptance in the agricultural field.
Probably the closest approach to a satisfactory solution to the problem of accomplishing a uniform distribution of a non-uniform mixture of agricultural fertilizers and chemicals over a fairly wide area behind a fertilizer spreader truck has been proposed in the patent to Palmer, U.S. Pat. No. 3,780,955. In this patent a material drops from the back of the fertilizer hopper onto the spreader apparatus which divides the material into a predetermined number of equal but separate streams. A spreader belt is projected transversely from one side of the vehicle body which is then divided transversely into sections corresponding in number to one-half of the material streams mentioned above. Each of the material streams thus formed is channeled to a corresponding transverse section on a spreader belt, after which the material is leveled to a uniform depth over the complete width of the spreader belt. The material is then progressively discharged to the ground at a constant rate in one direction transversely of a spreader belt and over the longitudinal length of such belt. Then another belt is disposed transversely of the vehicle body in the opposite direction and the mode of spreading mentioned is duplicated. The apparatus described in Palmer is shown as prior art at FIG. 1 and FIG. 2 of the drawings of this application, and the two principal drawbacks of this system are that the material thus spread must fall both through the top conveyor chain, which moves it outwardly for distribution, and then past a discharge regulating table, or device of some sort, and then again through the lower level returning chain. The second drawback, and the major one that has been present in the prior art from at least the time of the patent to Biszantz issued in 1951, is that the Palmer spreader leaves a large dead area (clearly shown in FIG. 2) behind the spreader truck where no material is deposited at all. It is totally unacceptable to retraverse the area that has been missed because it would "double" the materials already applied to the surrounding soil and would gain very undesirable results. This is a problem that has not been overcome by this type of fertilizer spreader device, and that is the principal reason that the rotating vane spreader device, shown in Hurt mentioned above, has been and remains to this day the common method of spreading agricultural particulates used in crop production on the farm fields of America.